The present invention relates to a method of cooling an optical fiber during drawing through contact with at least one cooling fluid in at least one cooling area.
There are various categories of optical fiber: optical fibers based on oxide glass, optical fibers based on fluoride glass, and plastics material optical fibers based on polymer materials. Optical fiber based on oxide glass, generally silica glass, is manufactured by drawing a heated preform, which is a large cylinder of silica glass, optionally at least partly doped, whose diameter generally lies in the range 20 mm to 200 mm and whose length generally lies in the range 300 mm to 2000 mm. FIG. 1 is a diagrammatic view of a drawing tower 1. A preform 2 is melted in a drawing furnace 3 which heats the preform to a temperature of approximately 2000xc2x0 C. A fiber 7 obtained in this way is cooled initially by the surrounding air, then in at least one cooling device 4, and finally by the surrounding air again, before it is fed into a coating device 5. The position of the cooling device 4 in the drawing tower 1 is generally optimized to obtain the correct fiber temperature for resin coating. The coating device 5 forms the coating of the fiber 7 from at least one coating resin which is usually cured by ultraviolet light. The device 5 generally includes at least one injection device (5a, 5c) followed by at least one curing device (5b, 5d). In the situation shown in FIG. 1, the device 5 includes a primary resin injection device 5a followed by a device 5b for curing said resin by ultraviolet light, and then a secondary resin injection device 5c followed by a device 5d or curing said resin by ultraviolet light. Finally, a coated optical fiber 8 is pulled by a capstan 6 and then wound onto a take-up spool 9.
The devices under the drawing furnace 3, which are on a common downward vertical axis Z, are generally identified by their position relative to the bottom of the drawing furnace 3, as indicated by the dimension z. All the components of the device shown in FIG. 1 are well-known to the person skilled in the art. Others, which are not shown, are also well-known to the person skilled in the art. Thus, for example, means for measuring the diameter of the bare and/or coated fiber, means for measuring the eccentricity of the fiber within its primary and/or secondary coating, and means for measuring the temperature of the fiber at a given distance along the axis are part of the prior art.
Cooling must reduce the temperature of the fiber leaving the drawing furnace to a temperature compatible with application of the coating resin, i.e. a temperature of the order of 50xc2x0 C. The temperature of the fiber leaving the drawing furnace is high, generally of the order of 1000xc2x0 C. to 2000xc2x0 C. for a silica-based fiber, depending on the drawing furnace and the drawing speed used. Cooling the fiber between leaving the drawing furnace and entering the coating device is one of the major problems to be solved in drawing fibers, especially if it is required to increase the drawing speed. It is known that the attenuation of the fiber depends on the cooling conditions, and moreover, if the temperature of the fiber on entering the coating device is too high, this can lead to problems both with the eccentricity of the fiber in its coating and with the quality of said coating. The speed at which silica-based fibers are drawn industrially, which was 300 meters per minute (m/min) a few years ago, has increased more and more, and is now of the order of 1500 m/min or more. This tendency is still apparent, associated with increasing productivity, which is one of the major objectives of the optical fiber industry.
The principle of the process for fabricating optical fibers based on fluoride glass is the same, but the preform is generally smaller, generally having a diameter of 15 mm to 20 mm and a maximum length of a few centimeters to a few tens of centimeters, for example 10 cm, and the temperature on leaving the drawing furnace generally lies in the range 300xc2x0 C. to 450xc2x0 C. The same technical problem can arise in this case. Similarly, the same technical problem can arise in the fabrication of optical fibers based on polymer materials, in which the preform is generally smaller, generally having a diameter of a few tens of millimeters, for example 80 mm, and a maximum length of a few tens of centimeters, for example 50 cm, and the temperature on leaving the drawing furnace generally lies in the range 200xc2x0 C. to 250xc2x0 C. The remainder of the description refers to optical fibers based on silica, but identical reasoning applies to other types of optical fiber, including optical fibers based on glasses other than silica.
Various devices have been used to cool silica-based fiber. One solution would be to increase the area of heat exchange between the fiber to be cooled and the surrounding air, in particular by increasing the distance between the drawing furnace and the coating device. However, this would entail increasing the height of the drawing towers currently used, which would be much too costly, especially in terms of the investment required.
Another solution is to improve the efficiency of cooling over the existing distance between the drawing furnace and the coating device. In addition to simple cooling by the surrounding air, which proves to be highly inadequate for the drawing towers currently used, the common principle of various devices used in the industry (as illustrated by European Patent Application EP-A1-0 079 186, for example) consists in injecting a gas radially towards the surface of the fiber at a given distance from the outlet of the drawing furnace and causing said gas to flow upwards or downwards over a particular length of the fiber, inside a heat exchange tube. As is well-known to the person skilled in the art, heat is transferred because of the thermal conductivity of said gas, which gas is generally air, carbon dioxide, nitrogen, argon, or helium, and is preferably helium possibly mixed with nitrogen. The periphery of the tube is preferably cooled by a cooling fluid, which is generally water. By way of example, U.S. Pat. No. 4,761,168 describes an improvement to such systems in which the gas is caused to flow along the fiber in a heat exchange tube of particular shape, which ensures regular renewal of the boundary layer of gas flowing along the fiber. The improvement is aimed at improving the efficiency of heat exchange.
One of the main problems encountered in subsequent use of optical fiber cooled in the above way is that the cooling imposed on the fiber during its fabrication, on leaving the drawing furnace and before passing through the coating device, significantly increases the level of Rayleigh back scattering associated with the fiber and therefore increases the major part of the attenuation of the optical fiber ready for use. It is known in the art that the attenuation of optical fiber at the wavelengths used, whether close to 1310 nm or to 1550 nm, must be as low as possible for optimum transmission of optical signals in said fiber.
That is why several solutions have been proposed to the problem of defining cooling profiles which are obtained by particular methods and/or devices and which minimize Rayleigh back scattering in the fiber. At least partial use of slow cooling profiles is generally proposed, meaning profiles that are slower than those obtained for cooling by the surrounding air. Patent Application DE-A1-3 713 029, for example, teaches slow cooling on leaving the drawing furnace.
Such methods are not satisfactory, however, in that they do not achieve sufficient reduction of the attenuation compared to the theoretical minimum attenuation, without degrading the mechanical strength of the fiber.
An object of the present invention is to alleviate the above drawbacks of prior art cooling systems by improving the cooling of an optical fiber during drawing. One particular object of the invention is to conserve or even to reduce Rayleigh back scattering significantly, compared to prior art cooling systems, and therefore to conserve or even to reduce the attenuation of the fiber fabricated by a drawing process using the cooling method of the invention, and to preserve the mechanical strength of the fiber.
To this end, the invention provides a method of cooling an optical fiber during drawing, through contact with at least one cooling fluid in at least one cooling area, wherein, in a fast cooling area, fast cooling, i.e. cooling that is faster than cooling in the surrounding air, from an initial temperature of the fiber lying in the range 2000xc2x0 C. to 1500xc2x0 C. for fibers based on silica glass, lying in the range 450xc2x0 C. to 250xc2x0 C. for fibers based on fluoride glass, and lying in the range 250xc2x0 C. to 175xc2x0 C. for fibers based on a polymer material, to a temperature at the end of fast cooling lying in the range 1700xc2x0 C. to 1200xc2x0 C. for fibers based on silica glass, lying in the range 400xc2x0 C. to 200xc2x0 C. for fibers based on fluoride glass, and lying in the range 225xc2x0 C. to 75xc2x0 C. for fibers based on a polymer material, is followed, in a slow cooling area, by slow cooling, i.e. cooling slower than cooling in the surrounding air, from a temperature of said fiber at the start of slow cooling lying in the range 1700xc2x0 C. to 1000xc2x0 C. for fibers based on silica glass, lying in the range 400xc2x0 C. to 150xc2x0 C. for fibers based on fluoride glass, and lying in the range 225xc2x0 C. to 50xc2x0 C. for fibers based on a polymer material, to a temperature of said fiber at the end of slow cooling lying in the range 1500xc2x0 C. to 700xc2x0 C. for fibers based on silica glass, lying in the range 350xc2x0 C. to 25xc2x0 C. for fibers based on fluoride glass, and lying in the range 200xc2x0 C. to 25xc2x0 C. for fibers based on a polymer material.
There can be a transition area in the surrounding air between the fast cooling area and the slow cooling area, although this is not essential.
The fast cooling of the fast cooling area is at least as fast, and preferably faster, than cooling in the surrounding air. In other words, the instantaneous slope dT/dt of fast cooling, where T is the temperature of the fiber and t is time, has a higher absolute value for such cooling than said instantaneous slope for cooling the fiber in the surrounding air. Said instantaneous slope has a higher absolute value for such cooling than said instantaneous slope for cooling the fiber in the surrounding air, preferably on average in the fast cooling area, more preferably in the major part of the fast cooling area, and even more preferably in virtually all of the fast cooling area.
The slow cooling of the slow cooling area is at least as slow, and preferably slower, than cooling in the surrounding air, at least locally, in the vicinity of the critical temperature. In other words, the instantaneous slope dT/dt of slow cooling, where T is the temperature of the fiber and t is time, has a lower absolute value for such cooling than said instantaneous slope for cooling the fiber in the surrounding air. Said instantaneous slope has a lower absolute value for such cooling than said instantaneous slope for cooling the fiber in the surrounding air, preferably on average in the slow cooling area, more preferably in the major part of the slow cooling area, and even more preferably in virtually all of the slow cooling area.
In a preferred implementation of the invention the temperature profile of each cooling area is established so that the fictive temperature of the cladding is maximized and the fictive temperature of the core is minimized.
An implementation of the above kind advantageously preserves the attenuation of the fiber during cooling whilst improving the mechanical strength of the fiber fabricated by drawing using the cooling method of the invention. The mechanical strength is defined by Bellcore standard CR 20, February 1998 edition, relating to the rupture of 50% of coated optical fibers subjected to longitudinal elongation, in a statistical distribution of fibers tested in this way, using Weibull""s statistical law.
The terms xe2x80x9ccladdingxe2x80x9d and xe2x80x9ccorexe2x80x9d are familiar to the person skilled in the art. The core corresponds to the part where at least approximately 50% of the light energy propagates, i.e. to the central part of the optical fiber. The term xe2x80x9ccladdingxe2x80x9d conventionally refers to the part which extends out to a diameter of 125 xcexcm, i.e. the peripheral part of the optical fiber complementary to the core. What is of interest in the present context is the peripheral part of the cladding, which influences the mechanical properties and in particular the mechanical strength of the optical fiber. The expression xe2x80x9cfictive cladding temperaturexe2x80x9d is used. In the case of an optical fiber made from a primary preform obtained by a modified chemical vapor deposition (MCVD) process, for example, surface-coated by deposition from a plasma, the relevant concept is the fictive surfacing temperature.
Fictive temperature is a concept recently introduced into the glass art with a view to better understanding the behavior of glass. The fictive temperature exploits the thermal history of a given silica compound, possibly one that has been doped, for a given property p. The concept can be applied to optical fibers, especially during drawing. At present it is measured indirectly on the material by Raman or infrared analysis. The present application refers to the fictive temperature associated with Rayleigh back scattering.
During drawing, the fictive temperature for said substance decreases continuously as a function of the imposed cooling profile, for a given substance, and assumes a constant value when the temperature of said silica is very much lower than its glass transition temperature.
The fictive temperature concept is the subject of much research. Reference is made here to the research carried out by G. W. Scherer, xe2x80x9cRelaxation in Glass and Compositesxe2x80x9d, Krieger Publishing, 1992, in conjunction with the work of C. T. Moynihan, xe2x80x9cPhenomenology of the Structural Relaxation Process and the Glass Transitionxe2x80x9d, Assignment of Glass Transition, ATM STP 1249, ed. American Society for testing and materials, 1994, pp. 32-49 and O. S. Narayanaswamy, Journal of the American Ceramics Society, 1971, 54(10), pp. 491-498. Programs based on the above research simulate fictive temperature calculations based on relaxation characteristics of glass with a given composition. Said simulations have been correlated with experimental results based on measuring the attenuation and mechanical strength of fiber heated and quenched under varying cooling conditions.
Because the core is the main light guiding part of the optical fiber, the attenuation of the optical fiber obtained by a drawing process during which the cooling method of the invention is employed is naturally related to the fictive temperature of the core of the fiber. Because of surface effects at the surface of the optical fiber, the mechanical strength of the optical fiber obtained by a drawing method during which the cooling method of the invention is employed is naturally related to the fictive temperature of the cladding of the fiber. The use of the method of the invention therefore results from a compromise between optimizing the fictive temperature of the core of the fiber and optimizing the fictive temperature of the cladding of the fiber.
The method of the invention therefore has the advantage of enabling practical control over two important parameters of optical fiber made by a drawing process during which said cooling device is employed, namely its attenuation and mechanical strength. It has been found that the fast cooling profile in accordance with the invention has virtually no influence on the attenuation of said optical fiber.
Another advantage of the method of the invention is that it is consistent with economic constraints which limit the height of the part of the tower available for cooling and which lead to the use of high drawing speeds. The fast cooling area represents an improvement in terms of the height of the drawing tower and/or the drawing speed, and enables the length of the slow cooling area to be increased, in other words fiber attenuation to be improved. It also has the advantage that the temperature of the fiber on entering the slow cooling area can be controlled, regardless of the drawing speed.
In one implementation, the profile of the slow cooling is chosen so that the fictive temperature of the core is minimized and the profile of the fast cooling is then adjusted so that the fictive temperature of the cladding is maximized. It is therefore possible to provide means for implementing the given slow cooling profile on the drawing tower on which the fiber is drawn, in order to minimize the fictive temperature of the core, and then to provide means for implementing the fast cooling profile on said drawing tower to maximize the fictive temperature of the cladding. An alternative approach, and one that is usually preferred, is to give priority to controlling attenuation, and to be content with the resulting maximum mechanical strength.
In another implementation, the slow cooling profile is chosen so that the fictive temperature of the cladding is maximized and the fast cooling profile is then adjusted so that the fictive temperature of the core is minimized. It is therefore possible to provide means for implementing the given fast cooling profile on the drawing tower on which the fiber is drawn, in order to maximize the fictive temperature of the cladding, and then to provide means for implementing the slow cooling profile on said drawing tower to minimize the fictive temperature of the core. An alternative approach that may be preferred is to give priority to mechanical strength and to be content with the resulting minimum attenuation.
The initial temperature of the fiber on entering the fast cooling area is generally approximately equal to the temperature of the fiber on leaving the drawing furnace. As previously explained, the temperature of the fiber on leaving the fast cooling area depends on the composition of the cladding and is established on the basis of the required fictive cladding temperature.
The initial temperature of the fiber on entering the slow cooling area and, as previously explained, the temperature of the fiber on leaving the slow cooling area generally both depend on the composition of the core and are established on the basis of the required fictive temperature of the core.
In one implementation, additional fast cooling is applied in a second fast cooling area after said slow cooling.
Additional fast cooling advantageously reduces the temperature of the fiber to the entry temperature of the coating area without degrading the mechanical and optical properties of the fiber.
The present invention, although described with reference to silica glass fibers, applies equally to the other categories of fiber described previously, the person skilled in the art adapting the temperature ranges accordingly.
The cooling fluid is chosen from the group comprising air, argon, nitrogen, carbon dioxide, and helium, for example. Said cooling fluid is preferably helium, possibly mixed with nitrogen.
The invention finally relates to apparatus for implementing the method of the invention including at least one given fast cooling device and one given slow cooling device.